Advances in mycobacterial genetics have made it feasible to alter virulent M. tuberculosis strains to create novel vaccine candidates. We have constructed mutants of the virulent M. tuberculosis strain H37Rv exhibiting diminished production of iron-manganese superoxide dismutase (SOD). These strains are tolerated better than BCG by mice and when administered as a vaccine, confer greater protection than BCG against subsequent infection with virulent M. tuberculosis, virtually eliminating the lung destruction that is characteristic of pulmonary tuberculosis. The focus of this application is to elucidate the mechanism of superior vaccine efficacy of our SOD-diminished H37Rv strains compared to BCG. We hypothesize that two major factors are involved. The first is the production of key antigens by H37Rv that BCG lacks. The second is a difference in the mechanism of antigen presentation, most likely apoptosis-associated MHC Class I antigen processing, that enhances the efficacy of the SOD-diminished H37Rv strain by stimulating a stronger immune response to antigens common to both BCG and H37Rv. The focus of this work involves elucidating the relative importance of these factors, with specific objectives that include: (1) To determine the direct effect of diminishing SOD production upon vaccine efficacy and immune correlates of protection; (2) To assess the importance of species and strain differences upon the vaccine efficacy of SOD-diminished M. tuberculosis complex strains including the amount of cross-protection generated against other M. tuberculosis complex strains; and (3) To determine whether modifying SOD-diminished BCG and SODdiminished M. bovis by adding specific antigens that are unique to M. tuberculosis improves vaccine efficacy against virulent M. tuberculosis. The findings from this work will help to optimize vaccine efficacy and may also expand our diagnostic capacity to distinguish prior vaccination from tuberculosis infection.